High-intensity X-ray and VUV sources are used in many fields: microscopy, materials science, biomedical and medical diagnostics, materials testing, crystal and nanostructure analysis, atomic physics, and lithography. These sources are the basis of the analytical base of modern high-tech production and one of the main tools in the development of new materials and products based on them.
The implementation of X-ray diagnostic methods requires compact, high-brightness X-ray sources, characterized by reliability and long lifetime. Depending on the applications, which include: visualization and 3D-reconstruction of the internal structure of organic and inorganic objects, high-contrast imaging of small organic objects, accurate determination of nanostructure parameters of materials—the spectrum energy should be in the range from 100 to 6 keV (from ˜0.01 to 0.15 nm), that is, in the range of hard X-rays. In this range, radiation is most effectively generated by direct conversion of electron beam energy into braking and characteristic radiation.
Obtaining radiation in soft X-ray (0.4-10 nm) and VUV (10-200 nm) ranges is most effective with the help of laser-produced plasma light sources. Their development in recent years has been largely stimulated by the development of projection extreme ultraviolet (EUV) lithography for high-volume manufacturing of integrated circuits (ICs) with 10-nm node and below.
EUV lithography is based on the use of radiation in the range of 13.5+/−0.135 nm, corresponding to the effective reflection of multi-layer Mo/Si mirrors. One of the most important metrological processes of modern nanolithography is the control of ICs for the absence of defects. The general trend in lithographic production is a shift from ICs inspection to the analysis of lithographic masks. The process of mask inspection is most effectively carried out with the help of its scanning by actinic radiation, i.e. radiation, the wavelength of which coincides with the working wavelength of the lithograph (the so-called Actinic Inspection). Thus, the control of lithographic mask defect-free production and operation is one of the key problems of lithography, and the creation of a device for the diagnosis of lithographic masks and its key element, the high-brightness actinic source, is one of the priorities of the development of EUV lithography.
The radiation sources for EUV lithography are using Sn-plasma generated by a powerful laser system including CO2 lasers. Such sources have the power of EUV radiation exceeding by several orders of magnitude the level of power required for the inspection of EUV masks. Therefore, their usage for mask inspection is inadequate due to the excessive complexity and cost. In this regard, there is a need for other approaches to the creation of high-brightness EUV sources for actinic inspection of EUV masks.
In accordance with one of the approaches known from the patent application US20020015473, published on Feb. 7, 2002, sources for the generation of high brightness X-ray or EUV radiation are known, including a liquid-metal-jet target supplied to the electron beam interaction zone.
Sources of this type are characterized by compactness and high output radiation stability. Due to the large contact area of the liquid metal with the cooling surface of the heat exchanger, a rapid decrease in the target temperature is achieved. Thus, it is possible to obtain a high energy density of the electron beam on the target and provide a very high spectral brightness of the source of X-ray or EUV radiation. Thus, liquid-metal jet X-ray sources have a brightness much higher than X-ray sources with a solid rotating anode and the use of liquid metal as a coolant, known, for example, from the U.S. Pat. No. 7,697,665, issued Apr. 13, 2010.
However, the circulation system of the jet liquid metal target is quite complex, which complicates the overall design of the radiation source. Also, these sources of radiation are characterized by the problem of contamination of the exit window, through which the beam of short-wavelength radiation is released. In the X-ray sources with a liquid-metal-jet anode, the intensive generators of debris are the nozzle and trap of liquid-metal jet, from which the fog from microdroplets of the target material spreads. As a result, the power of the radiation source decreases the faster the greater the power of the electron beam.
Part of this disadvantage is ameliorated in the high brightness liquid-metal jet X-ray source known from the U.S. Pat. No. 8,681,943, issued Mar. 25, 2014, in which an X-ray beam leaves the vacuum chamber through an exit window (preferably made of beryllium foil), equipped with a protective film element with a system of evaporative cleaning. The liquid metal preferably belongs to the group of low-melting metals, such as indium, tin, gallium, lead, bismuth, or their alloys.
However, the temperatures required for evaporative cleaning are high, e.g. about 1000° C. and more, for evaporation of Ga and In, which complicates the device.
Debris particles generated as a by-product during the operation of a radiation source may be in the form of high-energy ions, neutral atoms and clusters, or microdroplets of the target material.
The magnetic mitigation technique disclosed, for example, in the U.S. Pat. No. 8,519,366, issued Aug. 28, 2013, is arranged to apply a magnetic field so that charged debris particles are mitigated. In this patent the debris mitigation system for use in a short-wavelength radiation source, includes a rotatable foil trap and gas inlets for the supply of buffer gas to the foil trap so that neutral atoms and clusters of target material are effectively mitigated.
However, these methods do not provide highly effective suppression of the microdroplet fractions of debris particles in the path of the short-wavelength radiation beam. This limits the uptime of the equipment, in which the radiation source is affected due to the contamination of its optical elements.
Another debris mitigation technique, known from the U.S. Pat. No. 7,302,043, issued on Nov. 27, 2007, is arranged to apply a rotating shutter assembly configured to permit the passage of short-wavelength radiation through at least one aperture during the first period of rotation, and to thereafter rotate the shutter to obstruct passage of the debris through at least one aperture during the second period of rotation.
However, the complexity of using these debris-mitigation techniques in a compact radiation source means that technically they are too difficult to implement.
From the U.S. Pat. No. 9,897,930, issued on Feb. 20, 2018, it is known that a membrane from carbon nano tubes (CNT) having thickness more than 20 nm and high transparency for EUV radiation is used as a mask pellicle within the lithographic apparatus. It was proposed also to use CNT-membrane as a debris trapping system for EUV lithography source.
CNT-membranes are characterized by a number of advantages, including low cost and high strength, which allows them to be produced free-standing at large (centimeter) sizes, as is known, for example, from the publication of M. Y. Timmermans, et al. “Free-standing carbon nanotube films for extreme ultraviolet pellicle application”, Journal of Micro/Nanolithography, MEMS, and MOEMS 17(4), 043504 (27 Nov. 2018).
However, the use of a CNT-membrane for trapping debris particles generated by EUV lithography source is unlikely, as the CNT-membrane is highly likely to be destroyed by such powerful radiation. For less powerful sources of radiation, there is also a limitation. As our research has shown, a small fraction of debris particles with microdroplet sizes of more than 300 nm can penetrate through the CNT-membrane, which does not ensure the purity of the short-wavelength radiation source only through the use of a CNT-membrane.